C. Athanassopoulos, Harper College R. Oliver, Geosyntec G. Corcoran, Geosyntec

Design Considerations for Geosynthetics in Cover
Systems Over Mine Waste Rock and Tailings
C. Athanassopoulos, Harper College
R. Oliver, Geosyntec
G. Corcoran, Geosyntec
TAILINGS AND MINE WASTE 2014
Conference Sponsors
AMEC Earth & Environmental
Knight Piésold and Co.
Ausenco
MWH
BASF Chemical
MineBridge Software, Inc.
CETCO
Paterson & Cooke
ConeTec
Robertson GeoConsultants, Inc.
DOWL HKM
SRK Consulting, Inc.
Engineering Analytics, Inc.
Tetra Tech, Inc.
Gannett Fleming
URS
Golder Associates, Inc.
Community Sponsor
CDM Smith
TAILINGS AND MINE WASTE 2014
Introduction
• Waste rock dumps and tailings impoundments
are common features at mine sites.
• Many of these facilities contain sulfide-rich
minerals, which generate acid mine drainage.
• Control of AMD can be achieved by removing one
or more of the three essential components in the
acid-generating process: sulfides, air, or water.
Introduction
Engineered Cover Systems
• Possible cover systems
include:
 “Store-and-release” or ET
 Compacted clay or low
permeability soil
 Geosynthetic composite
covers
Design Considerations
• Hydraulic Performance
• Shear Strength and
Slope Stability
• Longevity and Maintenance
Water Balance Covers
• Temporary storage of precipitation in the soil
followed by removal of the stored water by
evaporation and vegetation transpiration
during dry periods.
• Can perform well, especially in drier climates.
• Advantageous from a cost-perspective if onsite materials can be used.
• Can be costly from a materials, design, testing,
and monitoring standpoint.
Compacted Clay Covers
• Traditionally used in mine closure and
reclamation projects to limit infiltration of
surface water into underlying waste.
• Clay source factors into cost.
• Thicker profile of 0.6 – 0.9 meters makes them
more difficult to puncture accidentally.
• Difficult to construct and are subject to
deterioration from various factors (differential
settlement, desiccation, freeze-thaw action).
Geosynthetic Composite Covers
• Geosynthetic barrier layer (geomembrane) placed over a
compacted clay or low permeability soil layer.
• The soil may be replaced by a geosynthetic clay liner (GCL).
• Commonly used geomembranes include HDPE, LLDPE, PVC,
fPP, CSPE, and EPDM.
• Drainage layer and cover soil are typically placed above the
geosynthetic barrier layer - provide lateral drainage,
protect the geosynthetics, and provide growth media for
vegetation.
Geosynthetic Composite Covers
• GCLs are considered equivalent to CCL with
respect to hydraulic and physical/mechanical
properties (Koerner and Daniel, 1993).
• Because GCLs are thinner, they are considered
more susceptible to puncture damage and
lateral squeezing during construction than a CCL.
• Installation damage can be limited with sound
construction practices and quality assurance.
USEPA ACAP Study
• Alternative Cover Assessment
Program (ACAP)
• 12 field sites nationwide
• Various types of covers (clay,
GCL, ET caps)
• GM/GCL composite liner
percolation
Source: Alternative Covers for Landfills, Waste Sites, and Mine Sites. Course Notes. April 1, 2010. Austin, TX.
USEPA ACAP Study
• GM/GCL composite cover percolation:
– Apple Valley, CA: 0 mm over 3 years
– Boardman, OR: 0 mm over 3 years
• GM/compacted soil cover percolation:
– Five sites, ranging from 0 to 2.6 mm/yr
• Compacted soil cover percolation:
– Three sites, ranging from 3.3 to 172 mm/yr (0.814.6%), and increasing with time, likely due to
desiccation, freeze/thaw, roots, etc.
• ET cover percolation:
– Twelve sites, ranging from 0 to 207 mm/yr (0-22.5%
of percolation), with highest values in humid and
sub-humid areas
GCLL Case study: Wisconsin Power Plant
• Ash monofill cover in Wisconsin.
• Lysimeter under a GCLL.
• Two phases: In the first phase, the GCLL was installed with
the geofilm downward. The geofilm was oriented upward in
the second phase.
• Percolation averaged only 2.6 and 4.1 mm/yr for the two
lysimeters over a five-year period (2001-2005), with no
signs of increasing.
• Represents approximately 0.5% of precipitation.
Source: Benson, Thorstad, Jo, and Rock. (2007), Hydraulic Performance of Geosynthetic Clay Liners in a Final
Landfill Cover, J. Geotech. and Geoenvironmental Eng. vol. 133, No. 7.
Modeling Geosynthetic Composite
Cover Performance
• Flow through a geomembrane is typically is very
low, it is considered impermeable.
• As a result, flow through a geosynthetic cover
system is evaluated assuming defects in the
geomembrane barrier.
• Can be evaluated using Giroud’s Equation (1997):
(
Q = Cqo 1 + 0.1 ⋅ (h t s )
0.95
)a
0.1
⋅h ⋅k
0.9
0.74
Slope Stability
•
•
•
•
•
Cover failures can be caused by:
Buildup of water in the cover
Excessive gas pressure beneath the cover
Excessively long or steep cover slopes or
Excavation at the toe of the slope (ITRC, 2003).
Challenge: driving forces ≤ resisting forces for
liner to be stable.
GM/GCLs on Slopes – Caps
Cincinnati Test Plots (1994)
• Cover soil/GC/GM/GCL/subgrade
• 5 plots at 3H-to-1V (18.4°)
• 9 plots at 2H-to-1V (26.6°)
Cincinnati Test Plots
• All 3H:1V plots have
remained stable for over 18
years
• Some of the 2H:1V plots slid
soon after installation:
– At interface between
geomembrane and woven
geotextile side of GCL
– Internal failure of
unreinforced GCL
• However, slides could have
been predicted by lab direct
shear tests and simple slope
stability analyses
Source: Bonaparte, R., Daniel, D.E., and
Koerner, R.M. (2002), “Assessment and
Recommendations for Improving the
Performance of Waste Containment Systems,”
EPA/600/R-02/099.
Infinite Slope Analysis
WITHOUT WATER, WITHOUT COHESION
tan δ
FS =
tan β
δ = critical (lowest) friction angle
β = slope angle
If FS ≤ 1.0, slide could occur
5
Calculated Factors of Safety
Interface
Peak Friction
Angle
FS on
3H:1V slope
(18o)
FS on
2H:1V slope
(26o)
Unreinforced GCL
/ textured GM
20o
1.1
0.7
Reinforced GCL
(woven side) /
textured GM
23o
1.3
0.9
Reinforced GCL
(nonwoven side) /
textured GM
29o
1.7
1.1
FS calculated using infinite slope analysis.
Assumes well-drained slopes (no pore pressures), and no cohesion.
See Daniel et al (1998).
Peak Interface friction angles (δ)
of GCLLs with various cover materials
GCLL
Silty
Sand
Internally Reinforced with
(20-mil textured geomembrane)
28 - 35o
Internally Reinforced with
(smooth geofilm)
18o
Clay
Gravel
36 - 39o 33 - 36o
19o
20o
Drainage
Geocomposite
31o
14o
Laboratory direct shear testing performed under low normal stresses (<400 psf),
representative of a cap.
Project-specific shear testing is always recommended.
Example Slope Stability Problem
Smooth and Textured GCLL/cover soil
Slope
(H:V)
Slope
(angle)
26.6o
FS with
Smooth film
GCLL
≤1
FS with
Textured GM
GCLL
1.1 to 1.6
2H:1V
3H:1V
18.4o
1.0 to 1.1
1.6 to 2.4
3.5H:1V
15.9o
1.1 to 1.3
1.9 to 2.8
4H:1V
14.0o
1.3 to 1.5
2.1 to 3.3
FS calculated using infinite slope analysis and δ = 18 to 20 degrees.
Assumes well-drained slopes (no pore pressures), and no cohesion.
Longevity
• Degradation methods for water balance covers
include biointrusion, cracking, and differential
settlement.
• Factors that can change the performance soil
properties include: Biointrusion, desiccation
cracking, erosion, puncture, freeze-thaw cracking,
differential settlement, etc.
• Improve longevity by using select soils,
placing/compacting at less than optimum M.C.,
or use thicker protective cover soil.
Longevity
• Service life of a geosynthetic cover is
dependent on the polymer chemistry,
exposure condition, and duration of exposure.
• Cover environment is ideal for maximizing
longevity of geosynthetics due to low chemical
aggressiveness, low overburden stresses, and
low temperature.
Longevity
• Muller et. al. (2008) identified the lower service
life of a GCL to be at least 250 years.
• A covered geomembrane will last much longer
than an exposed.
• Buried HDPE at 20˚C is predicted to last 446
years, at 40 ˚C - 69 years (Koerner et. al. 2011)
• Under exposed conditions, HDPE is expected to
have a lifetime of 36 years, PVC up to 18 years.
Conclusion
• While commonly used in final cover systems
for solid waste landfills for over 30 years,
geosynthetic covers have seen much less use
in cover systems for mine waste closures.
• Studies were presented highlighting
advantages and disadvantages of three
common cover systems, including hydraulic
performance, slope stability, and longevity.
Questions?